As compared to an acrylic-based or a rubber-based pressure-sensitive adhesive composition, silicone-based pressure-sensitive adhesive compositions are superior to the former in their electric-insulating, heat-resistant, weather-proof properties and adhesion to various substrates. Therefore silicone-based pressure-sensitive adhesive compositions find application in the structure of heat-resistant tapes, insulating tapes, heat-seal tapes, masking tapes for electroplating, masking tapes for heat treatment, etc. Depending on the mechanism of curing, the aforementioned silicone-based pressure-sensitive adhesive compositions can be divided into compositions cured by means of an addition reaction, compositions cured by a condensation reaction, or compositions cured by means of peroxides. From the viewpoint of curing properties, most preferable are compositions curable by means of an addition reaction.
Japan Patent Publication S63-22886 (Patent Reference 1) disclose that addition reaction-curable silicone-based pressure-sensitive adhesive compositions that consists of a diorganopolysiloxane having silicon-bonded alkenyl groups on both molecular terminals, an organopolysiloxane resin composed of R3SiO1/2 units (where R designates an alkyl group, an alkenyl group, or a hydroxyl group) and SiO4/2 units, an organopolysiloxane having in one molecule at least two silicon-bonded hydrogen atoms, a platinum-type catalyst, and an organic solvent.
However, adhesive tapes made with the use of the silicone-based pressure-sensitive adhesive composition disclosed in Patent Reference 1 are subject to delamination under the effect of heat and leave adhesive residues after peeling off from various adherends. Therefore, various silicone-based adhesive compositions have been proposed in order to solve both problems by increasing the adhesion force and at the same time reducing transfer of adhesive residue when the tape is used at high temperatures.
For example, Japan Patent Publication H04-335083 (Patent Reference 2) discloses a silicone-based adhesive composition that comprises an organopolysiloxane that contains vinyl groups and has a degree of polymerization equal to or greater than 2000, an organopolysiloxane resin composed of R3SiO1/2 units (where R designates a monovalent hydrocarbon group or a hydroxyl group) and SiO4/2 units), an organohydrogenpolysiloxane, a platinum-type catalyst, and a polysiloxane that contains vinyl groups and has a degree of polymerization equal to or lower than 2000. Same Patent Reference 2 discloses a method of preparation of the silicone-based adhesive composition by stirring under heating conditions a dimethylpolysiloxane having both terminals capped with hydroxy groups, an organopolysiloxane, and a methylpolysiloxane resin composed of Me3SiO1/2 and SiO4/2 units in toluene, then adding the remaining components, and mixing the entire composition (see Patent Reference 2, Practical Example 2).
Japan Patent Publication H10-110156 (Patent Reference 3) descloses a silicone-based pressure-sensitive adhesive agent comprising a product of partial condensation of a crude rubber-like organopolysiloxane containing in one molecule on average (a) one or more alkenyl groups and (b) an organopolysiloxane resin consisting substantially of R3SiO1/2 units (where R represents a monovalent hydrocarbon group) and SiO4/2 units, an organohydrogenpolysiloxane, and a platinum-based catalyst. The aforementioned composition is prepared by subjecting a crude rubber-like copolymer of a methylvinylpolysiloxane and dimethylsiloxane having both molecular terminals capped with silicon-bonded hydroxyl groups and a methylpolysiloxane resin consisting of SiO4/2 units and Me3SiO1/2 units containing 2.0 wt. % of silicon-bonded hydroxyl groups to a condensation reaction in xylene and in the presence of methylamine and then adding the remaining components to the obtained reaction product (see Patent Reference 3, Practical Example 1).
On the other hand, Japan Patent Publication 2002-275450 (Patent Reference 4) discloses a silicone-based pressure-sensitive adhesive composition comprising a cross-linked structure of a compound containing a silicone rubber and a silicone resin as main components. The silicone-based pressure-sensitive composition is characterized in that the gel fraction of the silicone-based pressure-sensitive adhesive composition rises in a range of 5-55 wt. % when heating of the adhesive composition is kept at 200° C. for 24 hr. The aforementioned silicone-based pressure-sensitive adhesive composition is prepared by stirring under heating conditions a crude rubber-like dimethylpolysiloxane and a methylpolysiloxane resin consisting of SiO4/2 units and Me3SiO1/2 units in toluene, then adding the remaining components, and further mixing the obtained mixture (Patent Reference 4, Practical Example 1).
Japan Patent Publication 2006-213810 (Patent Reference 5) also discloses an adhesive silicone composition that possesses excellent peel-off properties and does not leave adhesive residue even when used at a high temperature of 250° C. or higher. The composition is prepared by obtaining a reaction mixture from a polyorganosiloxane having in one molecule more than 2 alkenyl groups and a polyorganosiloxane consisting of SiO2 units and R13SiO0.5 units (where R1 designates monovalent hydrocarbon groups that may be the same or different and that contain 1 to 10 carbon atoms), and then combining the obtained mixture with a polyorganosiloxane having SiH groups, an inhibitor, and a platinum-based catalyst. An adhesive tape produced by curing the aforementioned composition on a substrate has also been disclosed.
It is known that the adhesive tape having an adhesive layer made from the aforementioned silicone-based pressure-sensitive adhesive composition provides high bonding/adhesive force and, to some extent, prevents the transfer of adhesive components to the substrate even when the tape is used during heat treatment at temperatures exceeding 200° C.
However, when these known adhesive tapes having the silicone-based pressure-sensitive adhesive layer is used for its heat-resistant properties and after the tape is peeled off, a residual microscopic amount of silicone can still be visually observed on the substrate surface, and an additional operation of cleaning the surface with an organic solvent becomes indispensable. Therefore, there was a demand for developing a pressure-sensitive adhesive that would leave a practically invisible amount of residual silicone component on the surface of a substrate so that it would be possible to eliminate the need for cleaning the substrate surface with an organic solvent, in particular when the aforementioned tape is used as a masking tape during a heat-treatment operation in solder reflow treatment of circuit boards.
With use of the silicone-based pressure-sensitive adhesive compositions disclosed in the aforementioned patent literature, it was impossible to limit development of the residual adhesive on the surface of the adherend after holding in a high-temperature environment at temperatures equal to or exceeding 250° C. and to get rid of the subsequent cleaning of the surface of the adherend with an organic solvent to an extent such that it would be possible to prevent discoloration of the surface of the adherend and to completely eliminate existence of the residual silicone on the adherend's surface.
Furthermore, above Patent References 1 through 5 do not teach the combined use of the following components: a condensation-reaction product between organopolysiloxane resin and organopolysiloxane having a high degree of polymerization; a non-reacted organopolysiloxane resin having a high degree of polymerization; and a non-reacted organopolysiloxane resin. Thus, it was not known that a silicone-based pressure-sensitive adhesive composition obtained by combining such condensation-reaction product with non-reacted organopolysiloxane and organopolysiloxane resin would almost completely prevent the generation of adhesive residue on the adherend's surface after peeling of the tape from it and would prevent transfer of the silicone components to the surface of the adherend.